I have experienced something similar. I used knoppix 3.3 (i think) and it would behave like yours; it would see the NIC in lspci, I would run netcardconfig but it would NOT get a address via DHCP, and it would not work even if I assigned it a static IP address. Since this was on a newly built PC, I had no idea if it was actually getting a address or not. I ended up using a hub and another PC running knoppix and I used ethereal to sniff the packets on my home lan. I discovered that the NIC was physically receiving the DHCP packets but it seemed as though the kernel wasn't processing them. So with the idea that the kernel might be the problem, I searched these forums and discovered that I could try to use a knoppix cheatcode noapic at bootup. I rebooted yet again and the noapic command worked! Now the NIC would behave normally and never exhibited the strange behavior.
This NIC problem has disappeared (for me) when I use knoppix 3.4 5-17.
I hope this helps.

James